Farmland habitat and Yellowhammer distribution in Britain

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Abstract
The relationship between yellowhammer density and a range of farmland variables was described, based on an analysis of 65 Common Birds Censuses conducted in 1965. Farm variables extracted from habitat maps submitted by census workers included altitude, area of arable, grassland and scrub, and length of hedgerow with or without trees while computations were made from CBC information for aspects such as number of bird species recorded on the farm, overall bird density and number of CBC visits made to the plot. The analysis consisted of the calculation of simple and partial correlation coefficients. The factors affecting yellowhammer density fell into 3 categories: those relating to bird density, those relating to hedgerow structure and those coincidentally correlated through cross-correlation with other variables. Nearly 1/3 of the variation in yellowhammer density was associated with variation in total density of birds on the farm; apparently yellowhammers are more numerous on highly productive land. About 1/6 of the variation in yellowhammer density was associated with hedgerow length and with the density of hedgerows per unit area. Hedgerows containing trees were much more important than those without trees, but there was no correlation between the amount of scrub or lines of trees. It is a combination of the hedge and tree components that is important to yellowhammers on farmland. Studies from individual CBC sites over different years confirm the results obtained from this broad-based analysis over many farms in 1 yr. One such study showed that the consistency of yellowhammer usage of individual fields over 9 yr was correlated with hedgerow density. A 2nd study showed the results of severe pruning of a farm''s internal hedgegrow system, in the following year terrritory size was substantially increased owing to the reduction in quality of the habitat.